Sunday's attack, which killed 17 people in a compound within earshot of Saudi royal palaces, came despite a tough six-month government campaign against Islamist fighters, triggered by triple blasts in the capital in May.
The fighters succeeded despite several warnings from Western countries that they were poised to strike.
Analysts say the latest attack drives home the gravity of the threat facing Saudi Arabia's rulers, who are grappling with the pressures of unemployment, a rapidly-growing population, a tide of religious and continued dependence on foreign workers.
"(The attack) is embarrassing to the Saudi authorities. They were supposed to be in control," said Magnus Ranstorp, a political analyst at St Andrews University in Scotland.
"This is very serious for the Saudis. For the Saudis this is an existential threat," he said.
The Islamist fighters had until recently a strong bedrock of support in the kingdom, observers say.
"This is very serious for the Saudis. For the Saudis this is an existential threat"
political analyst, St Andrews University, Scotland
But after the May bombings many Saudis began questioning the motives of al-Qaida leader Usama bin Ladin - who has denounced the Saudi government as an agent of the West.
As the cradle of Islam, the kingdom is a natural focus for Islamic fighters confronting Western powers in the Middle East, including US occupation forces in neighbouring Iraq.
"From a symbolic point of view, it's much more valuable for al-Qaida to create some kind of destabilisation or change in Saudi Arabia than to blow a bomb up in Sydney," said Sebestyen Gorka, fellow of the Terrorism Research Center in Virginia.
Geography is also on the side of the Islamists.
Saudi Arabia's vast deserts offer a bolthole for fugitive fighters, while the rugged 1460 km border with Yemen and the long Red Sea coast have for years been easy entrance points for armed groups and arms' smugglers.